The invention relates to a television pick-up device suitable for a discontinuous recording of information during continuous operation, the device comprising a pick-up tube having a cathode for generating an electron beam, a central electrode controlling the electron beam current strength, and a target plate which is scanned line-by-line and field-by-field by the electron beam by means of deflection means.
Such a device is disclosed in U.S. Pat. No. 3,602,641. The device, described by way of example of X-ray television, furthermore comprises a dark current compensation circuit. This circuit comprises a store for storing a given dark current setting, the value of which is determined during a field scan of the target plate when no light originating from a scene to be televized is incident on the pick-up tube target plate. It is described that the dark current information is recorded and processed after at least some field periods after a scene information recording, or video recording, has been terminated.
During a temporary interruption of the information recording, the information being either scene (or video) information or dark current information, the line-by-line and field-by-field scanning of the target plate can either be continued or be stopped. Stopping the scanning operation for some time has the drawback that during this period of time the target plate will carry an ever-increasing charge caused by the dark current in the target plate. This means that before a next information recording can be performed, the target plate must first be scanned during one or several field periods to remove the charge which is present due to the dark current. If, however, scanning the target plate is not stopped, the target plate would get a negative voltage with respect to the cathode; the stabilization level on the target plate shifts as compared to a normal continuous operation. The velocity distribution of the electrons in the beam is the cause thereof: the higher velocity electrons will be able to reach this plate in spite of a negative charge on the target plate, the negative charge being associated with an equilibrium state wherein the plurality of high-speed electrons which still reach the target plate are compensated for by the leakage, which corresponds to the dark current, in the target plate. At this dark current scan, the target plate may have in practice a voltage of, for example, approximately -1V at the free surface area which is scanned by the electron beam, instead of approximately the ground potential of OV which the electron beam has (on an average) in situ of the target plate. Since in the subsequent amplifier circuits, a picture signal generated by the pick-up tube is subjected to a dark current setting, which must be performed with respect to the ground potential which serves as a the black reference level, the signal information is clipped at the black level in response to the negative shift, which signal clipping may produce a loss of picture information and a reduced picture contrast, depending on an automatic gain control. To prevent negative charging of the free surface of the target plate the proposition might be put forward to increase the dark current artifically by irradiating the target plate by means of an additional, weak light source. This has the drawback that the additional light source must be able to irradiate the target plate uniformly, while local absorption differences in the glass wall onto which the target plate has been applied in the pick-up tube result in a non-uniform pattern.